Butadiene resonance structures

Resonance structure b is commonly used to describe delocalization of tt electrons in butadiene. Resonance structures c and d describe hyperconjugation. The structures represent electron transfer between anti hydrogens by ct-ct interactions. Note that the hydrogen atoms act as both electron donors and acceptors. As the two hydrogens are in very similar chemical environments, we expect littie net transfer of charge, but the delocalization affects such properties as NMR spin coupling constants. We will see in Topic 1.1 that this kind of delocalization is also important for hydrogens bonded to sp carbon atoms. 

These are resonance structures for the allylic cation formed when 1,3-butadiene accepts a prooton. 

Exercise 28-24 Why must the resonance forms 20a, b, c, etc. correspond to a singlet state Formulate the hybrid structure of a triplet state of butadiene in terms of appropriate contributing resonance structures. 

FIGURE 7.9 (a) Covalent resonance structures for butadiene and their overlap integral, (b) A VB mixing diagram of the resonance structures yielding the covalent ground and excited states, (c) The major 1,4-biradical character of the 2 Ag state. 

Figure 10. Schematic illustration of a tendency of each benzene fragment in naphthalene to retain its aromaticity by producing cis 1,3-butadiene partial localization in its twin-ring as described by the resonance structures (7a) and (7b) yielding the resulting predominant canonical structure (7c). This intuitive argument is supported by the (HF/6-31G ) bond distances and the corresponding 7r-bond orders given within parentheses.

A resonance argument can also be used to explain the shorter C—C o bond length of 1,3-butadiene. 1,3-Butadiene can be represented by three resonance structures ... 

Finally, 1,3-butadiene is a conjugated molecule with four overlapping p orbitals on adjacent atoms. As a result, the jc electrons are not localized between the carbon atoms of the double bonds, but rather delcx alized over four atoms. This places more electron density between the central two carbon atoms of 1,3-butadiene than would normally be present. This shortens the bond. Drawing resonance structures illustrates this delcx alization. 

In butadiene resonance occurs between the ordinary structure GH2=GH—GH=GH2 and other structures involving only one double bond, viz—GHg—CH=GH—GH,— and also, but to a lesser extent, +GHg—GH=CH— CH. The sum of the energies of the bonds in the ordinary structure is 778 4 kcals and the experimental heat of formation is 782 3 kcals the difference, which is the resonance energy is 4 I kcals. For i-methylbutadiene GH2—GH—GH=GHGH3, 616 kcals, for... 

A comparison of the resonance energies of structures involving triple and double bonds may be made by considering diacetylene, vinylacetylene and butadiene. Their resonance structures are as follows ... 

Resonance structures are used to convey the structural and electron distribution consequences of conjugation and delocalization. Let us look specifically at 1,3-butadiene,... 

In the diagram above there are two identical structures having opposite charge distributions and there is no net separation of charge. The importance of resonance structures to the composite structure increases with the stability of the individual structures, so structures B and C are less important than A, as they have separation of charge and only one rather than two tt bonds. By applying resonance criteria 3a and 3b, we conclude that these two structures contribute less stabilization to butadiene than the two equivalent benzene resonance structures. Therefore, we expect the enhancement of electron density between C(2) and C(3) to be small. 

The NPA electron distribution can be related to the VB concept of resonance structures. The orbitals corresponding to localized structures and those representing delocalization can be weighted. For example, Scheme 1.5 shows the relative weighting of the most important resonance structures for 1,3-butadiene, benzene, the benzyl cation, formamide, and the formate anion. These molecules are commonly used examples of the effect of conjugation and resonance on structure and reactivity. 

For methylenecyclopropene, a microwave structure determination has established bond lengths that show the strong alternation anticipated for a localized structure. The molecule does have a significant (1.90 D) dipole moment, implying a contribution from the dipolar resonance structure. The net stabilization calculated at the MP/6-31G level is small and comparable to the stabilization of 1,3-butadiene. The molecular geometry... 

Pentadiene, like 1,3-butadiene, has four tt electrons. However, unlike the delocalized TT electrons in 1,3-butadiene, the tt electrons in 1,4-pentadiene are completely separate from one another. In other words, the electrons are localized. The molecular orbitals of 1,4-pentadiene have the same energy as those of ethene—a compound with one pair of localized tt electrons. Thus, molecular orbital theory and contributing resonance structures are two different ways to show that the tt electrons in 1,3-butadiene are delocalized and that electron delocalization stabilizes a molecule. 

The central bond has some double-bond character, and the end bonds have some single-bond characte (In VB theory, this is explained by contributions from such resonance structures as CH2—CH=CH—CH2.) The sum of the bond orders is 5.235, exceeding 5. [This is because the 7r-electron energy of butadiene exceeds that of two isolated double bonds see Eq. (16.64).] For benzene each carbon-carbon bond order is found to be 5/3 = 1.667. 

Isomer 2 has a C-0 bond at the inner C atom of the butadiene skeleton and is unstable due to the non-resonance structure. Therefore, the nitrite radical is isomer 1, that is, the reaction sites are at the terminal C atoms of butadiene. The quantum yield for dissociation of nitrite radicals is much larger in alkene - NO systems than in those for... 

The animation shows the end carbon being protonated. Draw the intermediates that could result from protonation of each of the other carbons. Convince yourself that protonation at the end is preferred and that protonation at either end is equivalent in this case. Now go through this exercise with 2-methyl-1,3-butadiene. Draw each possible intermediate and decide which is most stable. Don t forget resonance structures ... 

Loss of an electron from ypt results in a cation in which the positive charge at the terminal position is clearly greater than at a 2-position. The same result is obtained from arguments based on resonance structures. For butadiene cation, we write the six important structures Vla-f. 

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